1. Field of the Invention
The invention relates to a method of generating test patterns during the application of solder paste to printed circuit boards by means of the screen-printing process, a structure being optically recorded as a reference pattern in a teach-in method step and this recording being used to generate reference data for the test patterns.
2. Description of Related Art
It is known within the functions of an inspection system to monitor the adequacy of the application of solder paste to printed circuit boards. The solder paste is applied to the printed circuit boards by the screen-printing printing process. It makes it possible in the subsequent component fitment process to solder electronic components to trace terminals formed on the printed circuit boards. This takes place in particular by the SMD process. To be able to check the application of the solder paste in the course of the inspection system, it is necessary to provide so-called test patterns. These define where and how the check on the application of solder paste is to be carried out. The test patterns define the coordinates, size and shape of the solder paste regions to be applied. To obtain test patterns, there are basically two known methods. The first method obtains the test patterns from CAD data. These are stencil data available in electronic form (Gerber files) of the printing stencil. These data provide information on where and how the printing stencil has openings through which solder paste is applied to the printed circuit board in the screen-printing process. Owing to problems with the availability, up-to-dateness and convertibility of the CAD data, the subsequently mentioned second method has been found to be the more practicable solution in the past this second method is a so-called teach-in method, in which the printed circuit board serves as a reference pattern for test pattern generation. Consequently, the trace and terminal structure of a printed circuit board is recorded by means of a camera and the test patterns are determined in this way in the teach-in method. One disadvantage is that the structures of tracesxe2x80x94as a result of their production processxe2x80x94have relatively great dimensional tolerances with the result that the generation of test patterns is subject to dimensional deviations. In particular in the case of contact structures which are very close together, for example for large-scale-integration components, the dimensional inadequacy may cause a problem.
The invention is therefore based on the object of specifying a method of the type mentioned at the beginning which produces high-precision results and can be carried out in a practicable way.
This object is achieved according to the invention by the printing stencil for the screen-printing process being optically recorded as the structure. Consequently, according to the invention, the printing stencil is used in the teach-in method as a reference pattern for the generation of test patterns. It will, of course, be understood by those skilled in the art that the reference pattern, which is made from the printing stencil before initial use, thus results in test data which correspond exactly to the intended conditions existing in the printing process, with the result that the print quality and dimensional stability can be optimally assessed independently of other influences. The measured print quality of the solder paste application allows direct conclusions to be drawn concerning the printing process, without disturbing factors occurringxe2x80x94such as the previously mentioned tolerance influences for example.
In order to generate the test patterns, preferably certain regionsxe2x80x94which are particularly susceptible with regard to any defectsxe2x80x94are specified and the test patterns are generated in these regions by means of optical recording of the structure (stencil openings) of the new printing stencil. The optically recorded image is converted electronically into data, which represent the test patterns.
These test patterns form the reference pattern, which is comparedxe2x80x94after the screen-printing processxe2x80x94with the likewise optically determined data of the actual solder paste application. This allows defects to be identified, for example, that two terminals of the printed circuit board are electrically connected by means of an undesired solder paste bridge.
It can also be identified whether solder paste is absent in certain regions. This in turn may have two reasons, on the one hand that the solder paste is sticking to the printing stencil, since the solder paste has not been transferred onto the printed circuit board, or on the other hand there is too little solder paste available, that is to say that it was not possible for the doctor blade to spread solder paste into all regions because there was insufficient available.
According to the previously known method, the solder paste application is optically recorded and compared with the test patterns obtained in the previous way. The comparison likewise takes place in the case of the subject of the invention, with the addition that the printing stencil is likewise optically sensedxe2x80x94because of the generation of the test patterns.
Consequently, both the printing stencil and the printed circuit board are assigned an optoelectronic recording device, which offers the following, previously unattainable advantages. It is possible to compare electronically, preferably automatically, the images obtained from the printing stencil and the printed circuit board, it being possible for defects or developing defects to be identified and possibly stopped in advance.
For instance, bridges usually only build up slowly, that is to say that it becomes evident on the printing stencil, in particular at the edges of the printing stencil openings, that particles of the solder paste are increasingly collecting there and building up over the course of time, until the formation of a bridge occurs. This progressive build-up is identified as it occurs from the optoelectronic sensing of the printing stencil and can therefore be stopped. This preferably takes place automatically, in that the printing process is briefly interrupted and a cleaning of the printing stencil is automatically carried out.
The same applies correspondingly to solder paste which sticks in the openings of the printing stencil and is therefore not transferred onto the printed circuit board, since in this case as well initially only slight amounts remain sticking to the stencil openings in a developing process and these amounts slowly build up until finally the paste clogs the entire opening or a large part of the opening. The optical sensing of the printing stencil is consequently used in two respects, on the one hand for the generation of the test patterns and on the other hand for the identification of defects. In addition to the simple generation of the test pattern by means of the procedure according to the invention, it is thus possible to carry out an optimum check with regard to possible defects during the screen-printing process.
It is preferably provided that the stencil openings of the printing stencil are optically recorded with regard to their position (coordinates) and/or their geometry (shape, size). The test patterns accordingly have corresponding information, that is on the position (coordinates) and the geometry (shape, size).
Furthermore, it is advantageous if the side of the printing""stencil facing the printed circuit board is optically recorded. The recording of the xe2x80x9cundersidexe2x80x9d has the advantage that it can best be identified therexe2x80x94in addition to the recording of the test pattern dataxe2x80x94whether defects can occur in the course of the progressing printing process, for example the bridge formation or sticking of solder paste mentioned.
According to a development of the invention, it is provided that, after the screen-printing process, an optical recording of the solder paste application which has taken place onto the printed circuit board is carried out and that actual data are generated from this recording. With the aid of these actual data, an assessment of the solder paste application is possible in the course of the inspection system in a particularly simple way, since it is merely necessary to carry out a comparison of the data of the reference pattern with the actual data. Consequently, an evaluation of the actual data is performed with the reference data taken into consideration.
The invention also relates to an apparatus for generating test patterns during the application of solder paste to printed circuit boards by means of the screen-printing process, having a first optoelectronic device for recording a structure as a reference pattern, taking place in a teach-in method step, and having data processing electronics for generating reference data from the reference pattern recording, the first optoelectronic device being arranged in such a way that it records the printing stencil as the structure. This optoelectronic device consequently obtains in an optical way, by sensing the structure (stencil openings) of the printing stencil, test pattern data, which are available as desired data in the evaluation of the solder paste application. These desired data are compared with actual data determined from the solder paste application. If there are deviations, corrections can be made.
It is advantageous if there is a displacing device, by which the first optoelectronic device can be made to enter between the printing stencil and the printed circuit board. This allows the respectively desired region of the printing stencil to be optically recorded.
There is advantageously provided a second optoelectronic device, for recording the solder paste application on the printed circuit board, it being particularly provided that the first and second optoelectronic devices can be moved by means of the same displacing device. This has the advantage that only one displacing device is required for both optoelectronic devices, with the result that structural complexity and costs can be reduced. The displacing of the optoelectronic devices by means of one and the same displacing device also has the advantage, however, that both devices are always brought into the same position with regard to the printing stencil and with regard to the printed circuit board, that is to say that the two optoelectronic devices always determine corresponding regions of the two parts, with the result that a desired-actual comparison is made possible in a particularly simple way.
Furthermore, it may be provided that the first and second optoelectronic devices are formed by only one optoelectronic apparatus, which can be switched over with regard to the recording direction or has two recording beams. Consequently, the two optoelectronic devices are combined in a single optoelectronic apparatus which is located on the displacing carriage of the displacing device. The complexity is reduced if the recording direction can be switched over, that is to say that the printing stencil andxe2x80x94after switching overxe2x80x94the printed circuit board are recorded by means of one and the same optoelectronics. To be able to perform a desired-actual comparison, it is then necessary that the image structure recorded first is stored and is then compared with the image structure subsequently recorded. Structurally rather more complex, but more advantageous from a process engineering viewpoint, is the configuration using two optoelectronic recording devices (cameras), one inspecting the printing stencil, by means of a first recording beam, and the other inspecting the printed circuit board, and consequently the solder paste application which has taken place there, by means of a second recording beam.
Furthermore, it may be provided that the optoelectronic apparatus has a first mirror, which has been made reflective on both sides and one side of which lies in the beam leading to the printing stencil and the other side of which lies in the beam leading to the printed circuit board. In particular, this first mirror which has been made reflective on both sides is in the form of a prism, which has the advantage that the plane of reflection is virtually the same for both beams.
A development of the invention provides that each side of the first mirror is assigned a semi-transparent, second and third mirror, respectively. The second and third mirrors are in each case assigned in particular first and second optics, respectively. The second and third mirrors are preferably in each case assigned a first and second image recording sensor, respectively. The arrangement is in this case preferably set up in such a way that the first optics lie between the second mirror and the first image recording sensor and the second optics lie between the third mirror and the second image recording sensor. The set up explained above makes it possible that the printed circuit board can be recorded via one side of the first mirror and the semi-transparent second mirror and also the first optics and with the aid of the first image recording sensor and that the printing stencil can be recorded with the aid of the other side of the first mirror, the third mirror and also the second optics and the second image recording sensor.
A first source of illumination interacts with the second mirror and a second source of illumination interacts with the third mirror, making it possible to illuminate the printed circuit board with the aid of the first source of illumination via the second mirror and the first mirror and to illuminate the printing stencil by means of the second source of illumination via the third mirror and the first mirror. Particularly good results can be achieved by the printed circuit board being subjected to light of an annular or approximately annular or part-annular third source of illumination. The third source of illumination is preferably located on the optoelectronic apparatus which can be moved by means of the displacing device. In particular, it is provided that, on account of its annular structure, the third source of illumination surrounds the beam between the first mirror and the printed circuit board, that is to say that optimum illumination of the printed circuit board takes place, with the result that the solder paste application on it can be optically recorded very well.
Finally, it is provided that the third source of illumination is formed by a plurality of light-emitting diodes (LEDs).